United States Department of Agriculture
Natural Resources Conservation Service
CEAP Conservation
Insight
Ephemeral Gully Erosion in
Cheney Lake Watershed,
Kansas
November 2006
Summary Findings
Background
•
About 10 percent of the watershed
contributes 76 percent of the sediment load to Cheney Reservoir.
•
Computer models can predict areas
of the watershed that have the highest potential for sediment loading.
•
Treating all ephemeral gullies in the
Cheney Lake watershed potentially
can reduce the sediment load to the
reservoir by 35 percent.
Cheney Lake Watershed (Fig. 1) covers
633,000 acres within five counties in
south-central Kansas. Over 99 percent
of the watershed is used for agriculture,
including small dairies, diversified crop
and livestock farms, rangeland, and
large acreages under center-pivot irrigation.
•
In this watershed, and in other watersheds with similar conditions,
focused voluntary implementation
of management practices that reduce
soil loss in the areas with the highest potential for sediment loading
will improve water quality more
rapidly and at less cost than random
implementation of practices across
the watershed.
The watershed drains into the North
Fork Ninnescah River, which flows into
Cheney Reservoir. The reservoir was
built in the early 1960s as a 100-year
multi-purpose project for water supply,
wildlife area, and flood control. It has a
surface area of approximately 15 square
miles with an average depth of 16 feet
and contains 168,000 acre-feet of water.
The City of Wichita draws 70 percent of
its daily water supply from the reservoir.
The two primary resource concerns for
the Cheney Lake Watershed are sediment and phosphorus pollution. Sedi-
ment is a concern with regard to water
quality and to reservoir storage capacity.
Phosphorus became a concern during the
early 1990’s, when Cheney Reservoir
experienced algae blooms significant
enough to produce taste and odor problems in Wichita’s water. The reservoir
currently is designated a high-priority
impaired water body under the Clean
Water Act, with impairments listed for
eutrophication and siltation.
A farmer-led Citizen’s Management
Committee created by the Reno County
Conservation District has implemented a
master plan for watershed pollution
management to reduce phosphorus and
sediment loading by 40 to 45 percent
and double the life of the reservoir. The
committee’s goal is to implement management practices on the land with little
or no cost to the landowner or operator.
A five-year comprehensive water monitoring study also was implemented in the
watershed, in 1996, through a partnership with the City of Wichita, the U.S.
Geological Survey (USGS), and the U.S.
Bureau of Reclamation. A two-year
macroinvertebrate study complements
the USGS water quality study.
As a Special Emphasis Watershed for
the Conservation Effects Assessment
Project (CEAP) the Cheney Lake watershed has been investigated by the Natural Resources Conservation Service
(NRCS) using the Annualized Agricultural Non-Point Source (AnnAGNPS )
computer modeling system. All AnnAGNPS modeling within this project has
been validated and calibrated to the
USGS data collected from 1996 to 2000.
Figure 1. — Location of the Cheney Lake Watershed in Kansas
This CEAP Conservation Insight presents significant findings regarding the
impact of ephemeral gullies. The NRCS
investigation also assesses the effects of
the Conservation Reserve Program, tillage practices, irrigation scheduling practices, concentrated animal feeding operations, atrazine management practices,
and the contribution of sediment from
ephemeral gullies.
Potential Sources of Sediment
Ephemeral gully erosion, which results
from concentrated water flow, produces
narrow, incised channels that cut down
to a less erodible layer. These temporary gullies may be obscured by tillage
(Figs. 2, 3), but they will reform in the
same location at the next rainfall event.
As soil is moved into the voided area by
tillage, an area wider than the actual
gully is damaged. Erosion loss from
ephemeral gullies -- beyond the calculated sheet and rill erosion loss -- is not
estimated by the USLE model and thus
is not included in national and regional
assessments of erosion control progress.
Examples of management practices that
will address ephemeral gully erosion
include grassed waterways or shaping
and seeding the affected area with permanent vegetation.
Stream bank erosion pertains to the loss
of soil from the incised banks of a perennial stream system. Stream bank losses
are best determined by a physical assessment of the stream system. Treatment
for stream bank erosion includes some
form of bank stabilization.
Figure 2. — Ephemeral gully in a crop field
in the Cheney Lake Watershed, August 26,
2005
Figure 3. — The same field on October 17,
2005, with ephemeral gully obscured by
tillage and planting operations
sheet and rill erosion. Estimates of
stream bank erosion were based upon a
1996 investigation by the Kansas NRCS
state geologist. A survey of the North
Fork Ninnescah stream system at that
time determined that less than 15 percent
of the sediment load originated from the
stream banks.
Upon investigation it was determined
that the USLE model did not capture
erosion as a result of ephemeral gullies
within crop fields. The hypothesis was
that ephemeral gullies would prove to be
the source of the sediments that were not
accounted for within the early modeling
studies. The AnnAGNPS model was
enhanced by adding a method to account
for ephemeral gully erosion. As a result,
current modeling results now closely
match the actual measured in-stream
sediment load.
When the AnnAGNPS model results of
the Cheney Lake watershed were validated with USGS stream monitoring
data, there appeared to be a gap between
the sediment load predicted by the
model and the actual sediment load
measured by USGS. Other outputs predicted by the model were in agreement
with the USGS data. Figure 4 shows the
difference in tons of sediment measured
by USGS and the sediment predicted by
the model.
With this modeling enhancement the
CEAP investigation has been able to
rank land area within the watershed according to its predicted sediment contribution to the overall loading of the
North Fork Ninnescah River. Using this
ranking, it is possible to see the relation-
25,000
USGS measured sediment
AnnAGNPS model simulation
without accounting for
ephemeral gully erosion
20,000
Sediment (tons)
Sources of sediment within this agricultural watershed include sheet and rill
erosion, ephemeral gully erosion, and
stream bank erosion. Sheet and rill erosion is defined as the removal of layers
of soil from the land surface by the action of rainfall and runoff. Sheet and rill
erosion can be erased by tillage practices, and the next erosive action may
occur in a different location. Typically,
sheet and rill erosion creates shallow,
parallel channels that are uniformly
spaced and sized. Estimates of soil loss
from sheet and rill erosion typically are
calculated using the Universal Soil Loss
Equation (USLE). Examples of management practices to address sheet and
rill erosion include conservation tillage,
gradient terraces, and field buffers.
15,000
10,000
Investigation of Sediment Origin
As watershed residents attempt to reduce
suspended solids within the Cheney
Lake watershed stream system, it is necessary first to determine the primary
sources of sediment. Early modeling
efforts in the watershed assumed two
primary sources—stream bank loss and
5,000
0
1997
1998
1999
Figure 4. — Comparison plots of observed and predicted sediment yield at the USGS gauging
station (07144870) on the North Fork Ninnescah River above Cheney Reservoir, 1997-1999
2
The relationship between sediment and
contributing cells can also be illustrated
spatially with the watershed map shown
in Figure 6. The purple-shaded areas,
based on AnnAGNPS model estimates,
make up the 10 percent of the watershed
that contributes 76 percent of the sediment. The green-shaded areas are those
areas that are contributing less than the
highest percent but higher than the mean
contribution.
Figure 7 shows the 989 points where
ephemeral gully erosion can originate,
Sediment load with gullies
Sediment load without gullies
100
35 % of the
drainage load
originated as
ephemeral
gully erosion.
90
Contributed Sediment Load (%)
ship between the percentage of sediment
load and the percentage of contributing
cells. The graph in Figure 5 illustrates
this relationship showing that roughly 10
percent of the 200-acre cells in the watershed are contributing approximately
76 percent of the sediment load. The
graph further illustrates the difference
between the sediment load when ephemeral gullies are included or excluded
from the equation. This graph indicates
that approximately 35 percent of the
sediment load in this watershed could be
eliminated by treating the ephemeral
gullies.
80
10 % of the drainage area produces
76% of the sediment load.
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
Contributing Drainage Area (%)
Figure 5. — Cheney Reservoir watershed sediment load, by unit area ranking ratio
Critical areas making up the 10 % of the
watershed area contributing the greatest
sediment load.
Other high priority areas contributing
above-average sediment loads.
Figure 6. — Sediment load contribution (model-generated) with respect to watershed outlet in Cheney Lake Watershed
3
according to the enhanced AnnAGNPS
model.
Conclusions — Implications and
Application in the Watershed
The finding that 10 percent of the watershed is contributing 76 percent of the
sediment load to Cheney Reservoir
could be used to make significant
changes in the implementation of conservation measures. A significant portion of the sediment source is ephemeral
gullies in tilled fields. If all ephemeral
gullies within the watershed are treated
with conservation practices specifically
designed to address this type of erosion,
the sediment load to the reservoir could
be reduced by 35 percent according to
the simulation model.
The broader implication for this watershed and for other watersheds with similar conditions is that voluntary implementation of conservation practices to
address specific sources of sediment in
specific locations within the watershed
will result in more rapid water quality
improvement than random voluntary
implementation of conservation practices. When funding and technical assistance are limited the ability to identify
and rank the pollutant sources will be
critical in focusing assistance in areas
that will provide the greatest improvements.
Figure 7. — Model-generated points where ephemeral gully erosion can originate in Cheney
Lake Watershed
—
The Conservation Effects Assessment Project — Building the Science Base
The Conservation Effects Assessment
Project (CEAP) is a multi-agency effort
to scientifically quantify the environmental benefits of conservation practices
used by private landowners participating
in U.S. Department of Agriculture
(USDA) conservation programs. Project
findings will help to guide USDA conservation policy and program development and help farmers and ranchers
make informed conservation choices.
Critical to building the CEAP science
base are the in-depth studies that address
specific resource concerns in representative watersheds nationwide. The research from these watershed studies enhances our understanding of conservation practice effects at the local landscape level and will provide guidance on
what practices are the most effective in
controlled non-point source pollution,
and where in the watershed these practices should be implemented to cost effectively address environmental impairment.
The Cheney Lake Watershed assessment
study is one of eight Special Emphasis
Watershed studies managed by the Natural Resources Conservation Service.
Other CEAP watershed studies are being
conducted by the Agricultural Research
Service and the Cooperative State Research, Education and Extension Service.
For more information:
www.nrcs.usda.gov/technical/NRI/ceap/
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